1. Field of the Invention
The present invention relates to a digital displacement measuring instrument that measures dimension and the like of an object based on axial displacement of a spindle.
2. Description of Related Art
A digital micrometer, one of general digital displacement measuring instruments, includes a body, a spindle slidably provided on the body, an encoder for detecting displacement of the spindle and a display for digitally displaying displacement of the spindle which is calculated from the detected value of the encoder.
The encoder has an opposing arrangement of a stator fixed onto the body and a rotor capable of synchronized rotation with the spindle, where the displacement of the spindle relative to the body is detected as a rotation angle of the rotor relative to the stator.
FIGS. 6 and 7 show a configuration around a rotor of a general digital micrometer. A rotor 41 is supported by a rotor bushing 44 that has an engaging key 43. A key groove 23 is axially formed on an external surface of the spindle 2 to be fitted with the engaging key 43.
A fixing member 51 that is fixed onto the body (not shown) and prevents the rotor bushing 44 from moving along the axial direction of the spindle 2 away from the stator 42 is provided on the opposite side of the stator 42, sandwiching the rotor bushing 44.
This arrangement helps the rotor 41 to stay at a predetermined position keeping a constant gap to the stator 42 regardless of the axial movements of the spindle 2.
The spindle 2 axially advances and retracts rotating in the circumferential direction of the body of the digital micrometer. Accordingly, since the rotor bushing 44 rotates in synchronization with the spindle 2, the rotation angle of the rotor 41 relative to the stator 42 is detected by the encoder to be converted to the displacement of the spindle 2 to be digitally displayed.
As above-mentioned, in the digital micrometer, the key groove 23 axially formed on the external surface of the spindle 2, the engaging key 43 that can be fitted with the key groove 23 and the rotor bushing 44 that includes the engaging key 43 are used in combination as a rotation transfer mechanism that transfers the rotation of the spindle 2 to the rotor 41 while keeping the constant gap between the stator 42 and the rotor 41.
However, in such a rotation transfer mechanism, there is a risk that a rotation transfer error occurs depending on depth position of the engaging key 43 relative to the key groove 23.
For example, as shown in FIG. 8, the fixing member 51 is occasionally hooked on the body, inclining against the spindle 2. Under such a condition, the rotor bushing 44 also inclines with respect to the spindle 2 while causing a problem of depth position of the engaging key 43 relative to the key groove 23.
FIGS. 9A and 9B show a fitting condition of the engaging key 43 and the key groove 23 when the spindle 2 is rotated under the condition where the fixing member 51 inclines with respect to the spindle 2.
Under the condition of FIG. 9A, the engaging key 43 is deeply fitted with the key groove 23. On the contrary, when the spindle 2 is rotated by 180 degrees, the engaging key 43 is shallowly fitted with the key groove 23 as shown in FIG. 9B. The depth position of the engaging key 43 relative to the key groove 23, thus, cyclically changes according to the rotation of the spindle 2.
The position of the engaging key 43 in the circumferential direction of the spindle also changes concurrently with the change in the depth position of the engaging key 43 relative to the key groove 23, which causes a rotation transfer error.
A digital displacement measuring instrument is proposed as a solution to such a problem, which presses an engaging key into a key groove by a plate spring to tightly fit the engaging key and the key groove constantly to prevent an occurrence of a rotation transfer error (for example, see: JP-A-2003-202201).
However, in the digital displacement measuring instrument as described in the above document, since a pressurization mechanism is required to be provided to press the engaging key into the key groove, number of parts accordingly increases and assembly of the parts becomes complicated. In addition, if too much biasing force is applied by the plate spring, the engaging key is overpressured into the key groove such that the spindle is prevented from smooth advancement and retraction.
To avoid this kind of problem, it is effective that the engaging key is adjusted to be at a depth position where the distal end of the engaging key contacts with the key groove while the engaging key is not overpressured into the key groove. However, this kind of adjustment is difficult since the depth position of the engaging key has to be highly precisely adjusted.